Adding the unit test for this plugin

@klaus modify the gemspec file to add the test-unit and test-unit-rr modify the Rakefile to run the test add the test/helper.rb to load the embulk for testing add the test/test_transaction.rb to test Attention: to test the plugin, the ruby version should be jruby, in my case, I use the jruby-9.1.12.0 run the 'bundle install --path vendor/cache' to install the related gems run the 'bundle exec rake' to build run the 'bundle exec rake test' to test